If i saw the card option i would assume they are agency as i doubt many independants carrying around card readers with them and i would assume the ones who would would be working for a company/agency...
It's wrong to assume that... there are high profile independents that would use the card payment via booking, probably via their website. Perhaps they use this service to cut down on the time wasters? Plus if you pay in advance, no awkward moment of handing over the money?